Cysteine or thiol proteases contain a cysteine residue at the active site responsible for proteolysis. Since cysteine proteases have been implicated in a number of diseases, including arthritis, muscular dystrophy, inflammation, tumor invasion, glomerulonephritis, malaria, and other parasite-borne infections, methods for selectively and irreversibly inactivating them provide opportunities for new drug candidates. See, for example, Esser, R. E. et al., Arthritis & Rheumatism (1994) 37, 236; Meijers, M. H. M. et al., Agents Actions (1993), 39 (Special Conference Issue), C219; Machleidt, W. et al, Fibrinolysis (1992), 6 Suppl. 4,125; Sloane, B. F. et al., Biomed. Biochim. Acta (1991), 50, 549; Duffy, M. J., Clin. Exp. Metastasis (1992), 10, 145; Rosenthal, P. J., Wollish, W. S., Palmer, J. T., Rasnick, D., J. Clin. Investigations (1991), 88, 1467; Baricos, W. H. et al, Arch. Biochem. Biophys. (1991), 288, 468; Thornberry, N. A. et al., Nature (1992), 356, 768.
Low molecular weight inhibitors of cysteine proteases have been described by Rich, Proteinase Inhibitors (Chapter 4, "Inhibitors of Cysteine Proteinases"), Elsevier Science Publishers (1986). Such inhibitors include peptide aldehydes, which form hemithioacetals with the cysteine of the protease active site. See, for instance, Cheng, H., Keitz, P., and Jones, J. B., J. Org. Chem. (1994), 59, 7671. The disadvantage of aldehydes is their in vivo and chemical instabilities.
Methods for selectively and irreversibly inhibiting cysteine proteases have relied upon alkylation by peptide .alpha.-fluoromethyl ketones (Rasnick, D., Anal. Biochem. (1985), 149, 416), diazomethyl-ketones (Kirschke, H., Shaw, E. Biochem. Biphys. Res. Commun. (1981), 101, 454), acyloxymethyl ketones (Krantz, A. etal., Biochemistry, (1991), 30, 4678; Krantz, A. et al., U.S. Pat. No. 5,055,451, issued Oct. 8, 1991), and ketosulfonium salts (Walker, B., Shaw, E., Fed. Proc. Fed. Am. Soc. Exp. Biol., (1985), 44, 1433).
Other families of cysteine protease inhibitors include epoxysuccinyl peptides, including E-64 and its analogs (Hanada, K. et al., Agric. Biol. Chem (1978), 42, 523; Sumiya, S. etal., Chem. Pharm. Bull. ((1992), 40, 299 Gour-Salin, B. J. et al., J. Med. Chem., (1993), 36, 720), .alpha.-dicarbonyl compounds, reviewed by Mehdi, S., Bioorganic Chemistry, (1993), 21, 249, and N-peptidyl-O-acyl hydroxamates (Bromme, D., Neumann, U., Kirschke, H., Demuth, H- U., Biochim. Biophys. Acta, (1993), 1202, 271. An additional summary of methods for reversibly and irreversibly inhibiting cysteine proteases has recently been compiled; see Shaw, E., Advances in Enzymology and Related Areas of Molecular Biology (1990), 63, 271.